Proper use of semicolon for crude emphasis

While there are multitudinous proper uses Of a semi-colon, its primary function is circumscribed by the type of punctuation the writer is assigning to the semi-colon. 1) the semi colon is a substitute for a period meaning that independent clauses must appear before and after the semi-colon. Hence your example may be acceptable and effective for prose, but it is grammatical incorrect as the second clause is dependent (a lot of people is the subject but a predicate (Verb) is lacking ... so no dice.

The other most common use of a semi-colon is as a surrogate comma, separating items in a list (following a colon). Typically the items in such a list are more complex than what can be expressed in a few words, hence they are more detailed and part of a program, process, components of an initiative or plan. The semicolon is a fancy comma or a learnéd period. Subtle, demure, and not common.

If you mean how would I rephrase the clause you originally posited, it's simply a matter of profiler punctuation and economy of language, as you already employed. Although your subject doesn't make sense since Cars don't kill people, rather other people driving cars.

My edit: Cars don't kill people; people driving cars kill other people. People, preoccupied with something beyond the steering wheel or in the blind spots among their rearview and side mirrors, kill people while driving with reckless negligence.

If you want to keep the original wording, I think an em dash is the most natural choice:

Cars kill people—a lot of people.

Em dashes are a good general-purpose way to introduce a pause (like a colon) or offset a phrase (like parentheses) where no other punctuation mark is quite right. An em dash tends to indicate more emphasis than a colon, a semicolon, or parentheses, which is a good fit for this case.

Your example isn’t a standard use of a semicolon, because the second part isn’t a complete phrase. Another punctuation mark such as a comma or colon would also work here, but may require rephrasing:

• Restating the noun for emphasis

  Cars kill a lot of people.

  ↕︎

  Cars kill people, a lot of people.

• Stating the topic, then explaining with more detail

  Cars kill over 30,000 Americans per year.

  ↕︎

  Cars kill people: over 30,000 Americans per year.

  (“A lot of” doesn’t fit in this case because it doesn’t add clarification, just emphasis.)

To me, using a semicolon fails on many levels. It fails grammatically (which is already understood), it fails conceptually (the two clauses are not tied together closely enough to warrant a semicolon), and it fails to deliver the desired level of emphasis.

For emphasis, use a separate sentence (or, as the case may be, fragment):

"Cars kill people. A lot of people."